1. Field of the Invention
This invention relates to rotary screen printing, and especially relates to rotary screen printing of textiles. It particularly relates to rotary screen structures for unsupported use in rotary screen printing processes and construction methods therefor.
2. Review of the Prior Art
Until recent years there were two main systems for applying patterns to textiles by printing, namely: (1) flat bed printing and (2) roller printing. While flat bed printing is relatively inexpensive, it is an inherently intermittent process which offers a rate of production much too slow to satisfy the textile industry. Moreover, in flat bed printing the overlap between screens on a flat bed machine can easily cause smudges and crush the still-wet pattern of the adjacent but overlapped newly printed surface. Such defect is evidenced as "cross-bars." Consequently, it is important that printing pastes be compounded so that they are absorbed by the fabric immediately on application; otherwise, crushing is likely to result. The rate of production of roller printing, however, provides a multiple increase over flat bed printing, which it has essentially superseded. However, roller printing also requires higher costs and capitalization and thus a demand for longer runs. Consequently, with advances towards automation of textile production, there has been a growing need for a system of applying patterns to textiles which does not require the high capitalization of the roller printing machine and the vast runs and the large amount of down time that are common to the roller printing process.
Rotary screen printing is a process which apparently offers a solution to the foregoing problems and which is being increasingly accepted by industry. The apparatus utilized in this system comprises a perforated cylindrical or rotary screen which is utilized to apply colored designs to the textiles in the form of an emulsion or paste which is forced from the interior of the cylindrical or rotary screen through the perforations of the screen and onto the textile work-piece in a pattern according to the engraving of the rotary screen, as described, for example, in U.S. Pat. No. 2,276,181 for both belt and cylindrical screens.
Generally, the screen is relatively thin, e.g., about 0.08 to 0.20 mm (about 0.003 to 0.008 inches). In operation, the colored emulsion or paste is passed from a reservoir inside the rotary screen by means of a squeegee which controls emulsion application, but it is the contact of the outer area of the screen on the textile substrate which determines the amount of pressure applied to the actual print.
While rotary printing evidences many advantages, there are problems with the rotary screen itself and with the methods of its fabrication. Some of the earliest rotary screens were made by mounting a suitable stencil fabric of silk or thin flexible metal over a printing cylinder, as described in U.S. Pat. No. 2,276,113. Such rotary screens were operated in a textile printing machine as described in U.S. Pat. No. 2,928,340. Other early rotary screens were fabricated from phosphor bronze mesh, made first as a flat screen and then soldered at the repeat join to form a cylindrical screen which is mounted on a printing machine while stretched between circular metal end rings, as described in U.S. Pat. No. 2,906,201. Such screens, however, were hindered by the soldered join and the susceptibility to corrosion of the metal from which the screens were fabricated. Large open patterns and continual stripes could not be printed, and the screens did not have a long life. Subsequently, woven cylindrical bronze or stainless steel sleeves were utilized, but these also met difficulties in usage and color application.
An improvement was represented by the Galvano method which involved the electrolytic deposition of metal onto a matrix of steel which had previously been impressed with a specific number of dots corresponding to certain mesh sizes. The dots on the matrix were filled with a dielectric. When the resulting matrix was placed in a plating bath, a sheet was produced having perforations corresponding in size and number to the non-treated dots. Such rotary screens, however, often damaged easily and wore out quickly due to the corrosive action of the colored printing emulsions and pastes.
Another improvement was represented by the use of nickel for deposition on the mesh, as described, for example, in U.S. Pat. No. 3,482,300. Because nickel is relatively inert to the chemicals encountered in the emulsions utilized, it became the chosen metal for such depositions.
Another rotary screen developed by the art was an all-metal screen fabricated by photographing the desired design image onto the circular matrix and depositing nickel thereon to a specifically defined thickness, the result being an all-metal sleeve containing the design and the necessary perforations.
Both of the latter types of rotary screens, however, are also easily damaged, e.g., by corrosion, especially by acid dyes. They are also readily dented; the imperfections resulting therefrom show up in the resultant printing and are particularly unsatisfactory and costly, especially when such damage occurs during the peak of a long printing run. Another disadvantage, inherent in the known rotary screens, is the cost of fabricating the rotary screen itself because of the cost of preparation or of engraving the screen, as well as the cost of the metal, e.g., nickel, utilized therein.
Another recent development, disclosed in U.S. Pat. No. 3,759,800, is an obviously costly combination of a metal sleeve, which is etched to produce a desired hole pattern, and a woven or knit fabric of polyester or nylon, which is heat-shrunk thereonto, the etched sleeve and the shrunk fabric being conjoined by electrolytically deposited nickel having a thickness equalling two-thirds of the depth of the threads. As this patent points out, a rotary printing screen must have a given degree of rigidity and inherent strength, in order to be practical and usable in commercial processes, and must be as thin as possible, in order to achieve fine printing quality. Clearly, precision in construction (i.e., uniformity in diameter) is also needed in order to obtain uniform printing pressure, as noted in U.S. Pat. No. 3,587,458, because non-uniform diameter creates an irregular squeegee pressure and consequent irregular printing results.
Accordingly, there exists a clear need for a seamless rotary printing screen having the thinness, extremely high bending modulus, resistance to denting, and precise construction that have hitherto been solely available in metal screens and the chemical resistance of a silk or plastic sleeve. However, such a combination of desirable characteristics is very difficult to attain because of the rigorous requirements of rotary screen printing, particularly in that the screen must be mountable and usable without support. Consequently, the rotary screen must possess a minimum tensile strength of 10,000 psi and a maximum circumferential variation of .+-. 0.1%. Ideally, the screen has a high folding endurance as measured by ASTM test D 2176-69. Thus, the material the screen is prepared from should exceed 100,000 cycles of this test.